The present invention relates to a control system for an automatic transmission for motor vehicles.
The automatic transmission for the vehicle operates automatically to provide a suitable gear ratio in accordance with vehicle speed and opening degree of a throttle valve of the engine. Further in such a transmission, a hold range including a first gear hold and a second gear hold is provided to hold the gear ratio in the first speed or second speed in order to ensure the power of the engine at uphill climbing or to effect the engine braking at deceleration of the vehicle. If the vehicle speed is accelerated in gear hold state, the engine speed is excessively increased, because of the large gear ratio.
Such a high engine speed may cause failure of the engine. In order to prevent such a trouble, a system is known in which a transmission is forcibly upshifted to prevent the engine from increasing to excessive speed, when the vehicle speed exceeds a predetermined value in the gear hold state.
On the other hand, Japanese Patent Application Laid-Open No. 51-54160 discloses a system in which actual transmission ratio is detected in accordance with the engine speed and the vehicle speed, an an optimum timing for changing the transmission ratio is obtained in accordance with the actual transmission ratio and detected engine speed or vehicle speed.
However, in conventional systems, since the excessive engine speed is detected based on the vehicle speed without considering other influences such as slip of a torque converter, inaccurate detection is liable to occur.